Broad-band optical communications typically require high-speed electro-optical modulators (EOM) to modulate light at a desired data rate. One common type of a broad-band EOM is a Mach-Zehnder modulator (MZM) that uses a waveguide Mach-Zehnder (MZ) interferometric structure with RF-driven optical phase modulators in each arm. The waveguide arms of the MZM are typically formed in an electro-optic material, for example a suitable semiconductor or electro-optic dielectric material such as LiNbO3, where optical properties of the waveguide may be controlled by applying a voltage to it. Such a waveguide modulator may be implemented in an opto-electronic chip as a photonic integrated circuit (PIC). A silicon photonics (SiP) platform based on Silicon on Insulator (SOI) technology may be particularly attractive for implementing broad-band modulators as it enables a natural integration with CMOS-based high-speed electronic drivers.
One common technique to high-speed modulation of propagating light, in particular at modulation rates on the order of 10-20 Gigabit per second (Gb/s) and higher, is the travelling wave approach, when the modulating electrical RF signals are applied to properly terminated electrical transmission lines that are electro-optically coupled and velocity-matched to the optical waveguides of the EOM. FIG. 1A schematically illustrates an example broad-band EOM in the form of an MZM 10 with two optical waveguide arms 11, 12 coupled to two electrical differential transmission lines 30 of length L, each formed by an inner signal electrode 22 and an outer signal electrode 21, with corresponding ground electrodes (not shown) and a differential transmission line termination 25. In the SiP platform, the electrodes 21, 22 may be overlaying p/n junctions formed across the waveguide arms that may either inject carriers (forward bias) or deplete carriers (reverse bias) in the waveguide core to modulate the refractive index of the waveguide by means of the carrier plasma dispersion effect. One known approach is a dual-differential modulation, in which each differential transmission line (TL) 30 is driven with a differential RF signal, so that in each differential TL 30 the inner electrode 22 and outer electrode 21 are driven with complementary single-ended RF signals, and the p/n junctions 31 in the waveguide arms 11, 12 are modulated in counter-phase; this effectively doubles the phase modulation amplitude at the output optical combiner of the MZM for a given peak-to-peak (PP) drive voltage Vpp applied to each electrode, as compared to more traditional implementations in which the inner electrodes 22 are grounded. The TL pairs 30 are configured so that the differential RF signals propagate along them at the same velocity as the light that is travelling in the waveguide arms 11, 12. In one common implementation, the PN junctions 31 formed along the length of the TL pairs 30 are reverse-biased, and may be referred to as depletion-mode high-speed phase modulators (HSPMs). However the dual differential modulator of the type illustrated in FIG. 1A may require two differential drivers, or a single differential driver of a double output power, to drive the two electrode pairs 30, which complicates the design. Additionally, further lessening the MZM driver power requirements is desirable in many applications.
Accordingly, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies for providing high-bandwidth optical waveguide modulators.